For most people, intuitive notions concerning probabilities are connected with relative frequencies of occurrence. For example, when we say that in toss ing a coin, the probability of its coming up "heads" is 1/2, we usually mean that in a large number of tosses, about 1/2 of the tosses will come up heads. Unfortunately, relative frequency of occurrence has proved to be an unsatis factory starting point in defining probability. Although there have been attempts to make frequency of occurrence part of the axiomatic structure of probability theory, the currently accepted formu1ation is one...